From the viewpoint of environmental protection, a polylactic acid resin using plants as raw materials has recently attracted attention. Main method for producing a polylactic acid resin includes a ring-opening polymerization method in which lactide as a dimer of lactic acid is polymerized through ring-opening, and a direct polycondensation method in which dehydration polycondensation is performed using lactic acid. While the direct polycondensation method is capable of producing polylactic acid at low cost as compared with the ring-opening polymerization method since lactic acid can be used as a direct polymerization raw material without undergoing the step of synthesizing lactide, there is a problem that the obtained polylactic acid resin has a low molecular weight. Therefore, there is disclosed a production method using direct polycondensation in combination with solid phase polymerization (Patent Literatures 1 and 2).
However, in case a solid phase polymerization treatment of a polylactic acid resin is performed, the step of performing crystallization in advance is required. Low degree of crystallinity of the polylactic acid resin may cause fusion between pellets of the polylactic acid resin during the solid phase polymerization treatment. However, the polylactic acid resin has a problem that it requires a long-term treatment so as to obtain sufficient degree of crystallinity because of its low crystallization rate. Therefore, various methods for improving the crystallinity have been studied (Patent Literatures 3 to 6, etc.).    [Patent Literature 1]    International Publication WO 2010/147176    [Patent Literature 2]    Japanese Unexamined Patent Publication (Kokai) No. 2010-209321    [Patent Literature 3]    Japanese Unexamined Patent Publication (Kokai) No. 2009-13352    [Patent Literature 4]    Japanese Unexamined Patent Publication (Kokai) No. 2009-24058    [Patent Literature 5]    Japanese Unexamined Patent Publication (Kokai) No. 2009-24081    [Patent Literature 6]    Japanese Unexamined Patent Publication (Kokai) No. 2009-67856